In duplicating an original using a flat bed scanner, the original is mostly a sheet. After the original sheet is placed on a contact glass surface of the scanner, a movable cover is placed over the original sheet to scan the original images. However, the original is not limited to sheets and includes bound materials such as books and booklets. The bound materials are also placed on the contact glass surface for scanning. As shown in FIG. 48, when a book original 100 is used, a bound portion 101 is lifted and not contacting the contact glass surface 102. Since the lifted bound portion 101 is away from the focus surface of the contact glass 102, the corresponding scanned image portion becomes inferior and includes distorted images, shadows, character distortions. Generally since the inferior image bound area 101 is hard to read, the character recognition (OCR) rate significantly degenerates. Particularly, a thick bound book causes a high rate of the OCR failures. To maintain the bound portion 101 on the focus plane 102 by pressing the book 100 onto the contact glass 102, one can damage the original book 100.
To solve the above described problems, prior art has proposed a method of correcting image distortion by speculating a three dimensional shape from the image density information. One exemplary speculation method for the three dimensional shape based upon the image density is as follows: T. Wada, H, Uchida and T. Matsuyama, “Shape from Shading with Inter reflections under a Proximal Light Source: Distortion-Free Copying of an Unfolded Book,” International Journal Computer Vision 24(2), 125-135 (1997). In the above reference, a method called, “Shape from Shading” is a typical example. Unfortunately, since the Shape from Shading method requires a long calculation time for the distortion correction process, the practical use of this method is difficult. Another prior art technology as disclosed in Japanese Patent Publication Hei 5-161002 includes a proposed method of correcting the distortion based upon the book shape that is measured by triangulation. Despite the simplicity, since triangulation requires a special shape measuring device for measuring the book shape, this prior art technology is also practical.
Yet another prior art technology as disclosed in Japanese Patent Publication Hei 11-41455 includes a proposed method of speculating a three dimensional shape of the book surface based upon the page outline shape of the scanned image. Although the amount of calculation is limited for the distortion correction, this prior art technology fails to effectively correct the distortion when the page shape is cut off and not completely included in the scanned image.
For the above shortcomings, a newly proposed distortion correction device effectively correct the distortions based upon a limited amount of calculation even if the page outline is cut off from the scanned image. Although the above newly proposed distortion correction device utilizes the information on character lines and rule lines rather than the page outline to correct the image distortion, the distortion correction is not perfect and still has some issues to be resolved. For example, when the rule line is broken due to fading, the rule line information is inaccurately extracted, and the correction accuracy is lowered. Furthermore, when the character line and photographic portions are erroneously identified, the character line information is inaccurately extracted. As a result, the correction accuracy is also lowered. The above described problems remain to be desired.